For projectors, the research and development of commercial products using a solid light source instead of a discharge lamp are actively conducted, aiming for improved performance and reductions in size and costs. For example, rear-projection TV sets and pocket projectors using an LED light source are commercially available.
For solid light sources for projectors, laser light sources are regarded as promising as well as LEDs. Although everyone recognizes the high potential of laser light sources for a light source, no projectors using a laser light source have yet been commercially available. The reason is that various restrictions are necessary because of the properties of laser beams, not to mention that inexpensive green semiconductor lasers are not yet in actual use.
For example, beam scanning type projectors that use a MEMS scanner to horizontally and vertically scan a laser beam for displaying images can be reduced in size almost beyond imagination, as compared with existing projectors. However, they need to comply with safety standards prescribed in the International Standard IEC 60825 or the like for safety of laser products. In the IEC 60825 classification, the light output of the light source is restricted to be small. Because of this, it is said to be difficult to achieve a brightness that is fully practical for projectors or to achieve a brightness that is equal to that of conventional projectors using a discharge lamp. In addition, because of the risk that laser beams can directly enter the human eye, safe illuminance is specified for individual classes, and the regulations vary depending of the conditions under which the human eye be exposed to laser beams.
On the other hand, a front projection type projector that indirectly scans a laser beam is known. For example, see JP2008-58454A. This is a type that directs a laser beam to a two-dimensional microdisplay such as a liquid crystal light valve or DMD (Digital Mirror device) and magnifies and projects images displayed on the microdisplay using an optical system such as a projector lens.
This type of projector is considered to implement brighter projectors as compared with the beam scanning type of projector.
Now, when a front projection type projector that uses a laser light source is operated, the state which poses the greatest risk is the state when the human eye is closest to the projector lens (projection lens).
In generally, because a microdisplay having a size that is about one inch or below is often used, it can be thought that the size of the light beam that passes through the part located on the emerging end side of a projector lens, i.e., the part with which people come into closest contact, is greater than a diameter of seven millimeters, which is the average size of the pupil of the human eye. Thus, the power of a laser light beam that has a diameter within the range of a diameter of seven millimeters can be considered to be sufficiently safe. Accordingly, the AEL (Accessible Emission Limit) that is calculated should be set to satisfy the safety class for the projector
In projectors using a laser light source, more particularly, in the case of front projection type projectors, a zoom lens is often used to a projector lens for the sake of user convenience and the advantages of the products. For example, in the case where a sufficient projector installation distance to match the screen size is not provided, an image area to be projected onto a screen can be matched with the screen size if the zoom magnification of a zoom lens is changed. In this description, the term “wide angle” means a setting state of the focal length f of the zoom lens in which the projection distance to a screen is made to be the shortest length when projecting an image area in a predetermined size, and the term “tele angle” means a setting state of the focal length f of the zoom lens in which the projection distance to the screen is made to be the longest length when projecting an image area in the same size as the image area of the wide angle.
Generally, in the zoom lens of the projector, the position and angle of a light beam passing through the surface of the light outgoing side of lens element are changed in the process of changing from the wide angle to the tele angle. Thus, the region, which is shining on the surface of the light outgoing side of the lens, (the area of the portion through which a laser light beam passes) changes according to zoom lens magnification adjustments. The size of this region is greater at the wide angle and smaller at “the tele angle” (see FIG. 1).
As a result, under the condition in which the laser output of the light source is constant, differences in safety, more specifically, differences in the AEL, occur between the wide angle and the tele angle.
It is the tele angle that is hazardous to the human eye because its laser power density is higher than that at the wide angle. For this reason, when the laser output is designed to meet the AEL or below which satisfies a safety class at the tele angle, the safety class can be compensated for the entire variable power region of the zoom lens. However, as regards these measures, the safety margin potential according to the AEL is not optimized. In other words, although much brighter images can be lowfully projected at the wide angle, the brightness is not increased.